Electromagnetism, Properties Of Waves, Motion, Gases & The Kinetic Theory Worksheet With Answers Page 19
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36Momentum
Momentum is a difficult thing to explain – simply, it is how much ‘motion’ an object
has. However, it is quite easy to calculate the momentum, p, of an object if you know
the object’s mass, m, and velocity, v, (velocity is the vector version of ‘speed’). This is
the equation for calculating momentum :
p = m x v
momentum = mass x velocity
Docked !!
p = m x v = 3 000 x 10
p = m x v = 70 x 5
p = m x v =
x 0
50 000 000
= 30 000
= 350
= 0 (zero !)
kgm/s
kgm/s
kgm/s
Here’s the Law of Conservation of Momentum :
The total momentum of a system of interacting bodies is constant provided
there are no external forces acting.
rd
This law is perfectly consistent with Newton’s 3
Law ! Take a look at the imminent collision
below :
Car A
Car B
rd
As they collide, car A will create a force to the right () on car B. Newton’s 3
Law states
that car B will therefore produce an equal but opposite force on car A to the left ().
nd
We need Newton’s 2
Law too !
p
where p = change in momentum
F =
t
F x t = p
Re-arranging
Since the cars are in contact with each other for the same amount of time, F x t will have
the same value for both cars, and hence, p will have the same value for both cars – this is
‘conservation of momentum’ since any momentum lost by car A will be given to car B.
(Remember that momentum is a vector, and so ‘positive momentum’ () from car A will
seem to ‘cancel out’ some of car B’s negative momentum !)
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